This invention generally relates to memory systems, and specifically uses ferroelectric films to create memory systems that are nonvolatile. The invention demonstrates impressive volumetric density, random access capability, and bandwidth and access times that far exceed what is possible with current rotating magnetic media.
Ferroelectric materials, such as BaTiO3 and SrTiO3 are known to possess a low symmetry that allows them to be polarized by an electric field, as shown in FIG. 1a. As used herein, the term xe2x80x9cferroelectric materialxe2x80x9d refers to any material in which polarity can be permanently set by means of application of an electric field, and encompasses the specific materials enumerated above and any other materials having the described properties.
The polarization of ferroelectrics increases as the voltage across the film is increased. When the electric field passes a saturation-polarization threshold, the film remains permanently polarized in that direction, and will decay to a fixed remnant polarization some time after the voltage is removed. This phenomenon is shown in FIG. 1b. 
Threshold voltage depends on the thickness of the film. For a typical 0.2 micron thin film, 3-5 Volts is quite ample to effect a change in polarization in either direction for most ferroelectric films.
FIG. 2 shows that there is a relationship between physical pressure on a polarized film 22 (where the direction of polarization is indicated by arrows 20), and induced voltage. When a polarized film is compressed, for example by a distance xcex94Z as shown in FIG. 2, a small displacement current through the film is generated in the compressed film. This results in charge accumulating at the surfaces of the film (e.g., on metal plates) which is the manifestation of the voltage induced by the applied pressure. Whether the voltage is positive or negative depends on the polarity of the film.
This mechanism also works in reverse. Particularly, if a voltage is applied across a polarized film, the film expands or contracts depending on, the magnitude of the voltage, and whether the voltage is positive or negative with respect to polarization. This effect is characterized by the ferroelectric""s piezoelectric coefficient, which is in the range of 200 pC/N for the typical kinds of ferroelectric films in use today.
It is an object of the invention to provide a memory cell in which the state of the cell can be determined by applying a mechanical force to the cell and reading an induced electrical charge therefrom.
It is another object of the present invention to provide a nonvolatile memory system using ferroelectric films having a first film used to store a polarization state, and a second film used to apply mechanical pressure to the first film for the purposes of sensing the stored polarization state.
It is another object of the invention to integrate the aforementioned films onto a silicon surface so that circuitry in the silicon can be used to store polarization states, and to sense polarization states.
It is another object of the invention to use the aforementioned circuitry to provide random access to the stored states in the aforementioned ferroelectric film.
It is another object of the invention to use the aforementioned integrated silicon structure, henceforth referred to as a chip, to provide a wide parallel output which effects a high output bandwidth.
It is another object of the invention to provide periodic outputs from this chip by vibrating the aforementioned second film at its natural resonant frequency so as to operate the chip efficiently.
It is another object of the invention to create a memory system using two chips that share the aforementioned resonating structure by stacking the chips face to face, one on top of the other.
It is another object of the invention to create a larger memory system using a plurality of aforementioned memory systems by stacking the aforementioned memory systems on top of each other to obtain a large volumetric density, and even wider bandwidth.
It is another object of this invention to create an even larger memory system from a plurality of aforementioned larger memory systems in parallel.